Phil Mickelson, after taking "a few days to calm down," said he's "embarrassed and disappointed" after hitting a moving ball on the green during the U.S. Open.

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Mickelson tamped down fescue grass with his foot in front of the seventh tee Sunday at A Military Tribute at The Greenbrier in West Virginia. He then called a 2-stroke penalty on himself for improving his line of play.

At the U.S. Open last month, Mickelson intentionally violated golf rules by hitting a moving ball on the green in the third round. He later apologized, saying his anger and frustration got the best of him.

On Sunday's gaffe, Mickelson said he "wasn't really thinking." After stepping on the grass, he paused before he hit his tee shot, realized the mistake and checked with a rules official, who confirmed the violation.

Mickelson shot 74 on Sunday to finish 1-under 279 for the tournament, which was won by Kevin Na.